This invention relates to illuminators and more particularly to illuminators for use in machine vision systems for inspection of irregularly shaped surfaces.
It is often desired to inspect objects during manufacture for imperfections, and to verify proper size, shape and surface characteristics. Such inspections can be part of an overall quality control process undertaken during manufacture. The inspection procedure can be performed using automated equipment, or by an operator who visually inspects each part as it passes through an inspection station. Parts are typically transported to and from the inspection station by a moving conveyor belt, robot arm or other transport mechanism. Surface imperfections, and other irregularly shaped formations on a part, may extend transversely to the flat surface under view-thereby requiring specialized illumination to be properly viewed. In general, such specialized illumination is generated by a diffuse lighting source.
FIG. 1 shows a generalized illumination system for inspecting irregularly shaped parts according to the prior art. The system 10 includes a base 12 upon which a part to be inspected 14 is located. The upper supporting surface 15 of the base 12 can have a variety of shapes, colors and reflectivity depending upon the inherent optical characteristics of the part 14. In many systems, the surface 15 is matte black in reflectivity and color. The base 12 can be a moving surface such as a conveyer belt or other transport mechanism for carrying parts through the inspection system 10, as noted above.
The part 14 is located on the surface 15 directly beneath a combination light source/reflector assembly 16. The clearance C between the bottom rim of the reflector assembly and the surface 15 is usually sufficient to enable the part to pass thereunder without requiring axial (up and down) movement of the reflector assembly 16 or base 12. The inner surface 18 of the reflector assembly 16 defines the interior of a hemisphere in a typical system arrangement. The surface 18 is coated with a matte white coating to generate a highly diffuse reflected light from any light striking the surface. The illumination light is, itself, provided by a ring illuminator 25 constructed integrally within the reflector assembly. Specifically, the bottom edge 20 of the reflector assembly 16 includes a well 22 with an inner upstanding wall 24. This well, defining an annular channel, houses the light elements of ring illuminator 25. The ring illuminator 25 is generally formed from a series of LEDs 26 arranged at even intervals around the perimeter of the edge 20. Fiber optic tips or other light sources can also be employed. The ring illuminator 25 generates a series of rays (shown schematically as arrows) 30 that strike the part 14 in an indirect manner. As such, the exposed surface of the part is illuminated effectively with a largely diffuse, indirect light.
During inspection, the illuminated part 14 is viewed through an aperture 32, located at the apex of the hemisphere. In the typical system a camera assembly 34 is mounted at an appropriate focal length above the aperture for remote viewing of the part. The camera assembly 34 can comprise any acceptable, commercially available solid-state camera unit usually adapted to transmit real-time image data of viewed subjects. The camera assembly 34 can include a telecentric lens arrangement 35 according to a conventional design. This type of lens selects light rays that are oriented generally parallel to the axis (dashed line 37) of the camera 34. The lens can include a zoom function and inherent magnification as well as internal aperture (F-stop) control.
A typical camera assembly for use in the illustrated system employs a solid state CCD element that generates an image signal for transmission via a data line 36. This image signal is carried over the line 36 to an image processor/frame grabber 38. The image processor/frame grabber 38 typically acquires a single image or set of frames showing the viewed part, and may be programmed to provide digital enhancements to various portions of the image. The captured image is displayed on a video display 40. Various input/output (I/O) peripherals can be provided to control the image processor/frame grabber 38. These can include various microprocessor functions accessed through a keyboard, mouse or graphical user interface. As noted above, the process can be automated.
The above-described inspection system 10 has the disadvantage that the ring illuminator 25 is located at a significant distance (clearance C) above the part 14. In addition, the upstanding wall 24 of the illuminator housing further blocks illumination light at lower angles of incidence with respect to the part 14 and underlying surface 15. The higher-angle rays 30 shown are less capable of fully illuminating side edges (for example edge 44). As such, structures and edges on the part that require low-angle illumination for proper definition are less well-defined.
A further disadvantage of the illuminator according to FIG. 1 is that the size and shape of the reflector assembly 16 limits the variety of illumination structures that can be employed as well as the color and intensity of the illuminator. In addition, the reflector must include various electrical connectors and leads when incorporating an illuminator therein. These overhead connections and leads can be cumbersome, and can sometimes impede movement of parts and equipment about the manufacturing floor.
Accordingly, it is an object of this invention to provide a method and apparatus for illuminating parts with a uniform diffuse light that enables lower-angle light rays to strike the part. This invention should decrease the complexity of the reflector by removing various elements for integral illuminators and should enable a wider range of illumination colors, intensities and light characteristics to be employed.
This invention overcomes the disadvantages of the prior art by providing an illuminator in the form of a backlight unit upon which the part is directly supported while a hemispherical-shaped reflector is positioned above the part, free of a ring illuminator thereon, so that light is reflected onto the part at a lower angle with respect to the supporting base. This more-fully illuminates side edges and upright surfaces on the part for enhanced viewing of these features. An aperture is located at the apex of the reflector, overlying the part. A camera or other viewing device is positioned over the aperture. Through the use of a plane polarizer on the backlight and on the camera lens, any light transmitted directly from the backlight to the camera is filtered-out, while indirect light bounced from the reflector back onto the part is substantially fully viewed.
In a preferred embodiment, the edges of the reflector are brought as close as possible to the level of the part without obstruction by the upstanding walls of an integral illuminator. As such, lower-angle incident light is effectively transmitted onto the part for viewing by a camera. This camera is mounted above an aperture at the apex of the reflector. The reflector can define a hemisphere with a matte white inner surface for providing a highly diffuse light. Alternatively, a different geometry can be employed and/or a different reflector surface characteristic is contemplated. The backlight unit, according to a preferred embodiment, comprises a commercially available structure having an upper diffuser plate and a grid of upturned fiber optic tips mounted therebeneath to generate a wide field of diffuse, uniform light. In particular, the tips are formed on the ends of a series of respective fibers that are collected into a cable bundle. The cable is itself connected to a high-intensity light source. Various filters, controllers and selected bulb intensities can be used to vary the characteristic of light provided to the backlight unit by the light source. A variety of part transport mechanisms including conveyer belts, trolleys and robot arms can be incorporated into the system of this invention. The reflector is located so that it provides clearance for parts to pass into and out of the inspection area thereunder.